The staves and other items have magical properties which allow mages to channel their energy. The swords are just swords. So if you equip "just a sword" then it stands to reason that your THM skills would be hampered. I outlined above how they could add specialisations within the classes to accommodate the things you want.
Ignore it if you want, but Yoshi P already said there are two options they are going for: Advanced classes, which essentially would keep all the current classes. The other route is to make classic jobs specialisations, which would kind of replace the current classes while maintaining the armory system.
Perhaps they will add magical swords which can heighten your magical potency. Or perhaps a specialisation within the classes can help facilitate a sword-wielding mage to make it viable.
The thing i dislike about your proposal is that you are asking for THM to be a specialisation in and unto itself, which is worse, in my opinion than wanted specialisations within the THM class. I don't like the current classes because they lack direction, hence why I am encouraging specialisation within the classes.
You are suggesting they make THM a role/job instead of a class.